1. Field of the Invention
The present invention relates to analog to digital (A/D) data converters. More particularly, the present invention relates to wide dynamic range A/D converters.
2. Description of the Related Art
A digital signal processing type hearing aid must include an analog to digital (A/D) data converter, to turn the analog signal from the receiver into a digital signal prior to processing. Such an A/D converter must have low power requirements, and must also handle signals having a wide dynamic range. Speech signals might have dynamic ranges exceeding 80dB.
The preferred A/D converter for audio in general, and hearing aids in particular, is based upon a delta sigma modulator. Delta sigma modulation incorporates a noise-shaping technique whereby the noise of a quantizer operating at a frequency much greater than the bandwidth is moved to frequencies not of interest in the output signal. A filter after the quantizer removes the out of band noise. The resulting system synthesizes a high resolution data converter, but is constructed from low resolution building blocks. A good overview of the theory of delta sigma modulation is given in "Oversampling Delta-Sigma Data Converters," by Candy and Temes, IEEE Press, 1992.
A delta sigma modulator generally comprises one or more integrators for the input signal, which are fed into a quantizer, the output of which is the output of the delta sigma quantizer, and is also fed through a digital to analog converter in a feedback loop to the integrators. In the case of an A/D converter, the integrators are time sampled analog, usually switched capacitor, and the output is a digital bit stream to a decimation filter. In a delta sigma converter, there are three major factors which contribute to dynamic range, the order of the loop (the number of integrators), the number of levels of the quantizer, and the over sample ratio. In practice, delta sigma modulators are generally at least second order, because higher order modulators better reduce noise in the signal band, due to improved prediction of the in-band quantization error. Thus, the resulting signal to noise ratio is better. Second order delta sigma modulators are relatively stable, and easy to design. U.S. Pat. No. 5,392,042 describes how to build high order modulators for higher precision. U.S. Pat. No. 5,461,381 provides a good reference on implementation details of switched capacitor sigma delta converters.
Conventional, one bit, delta sigma converters must be clocked at around 2 MHz to achieve the desired wide dynamic range for audio signals. Such high clock speeds require more power than is desirable in a hearing aid. In order to achieve the desired dynamic range for a hearing aid at lower clock speeds, the quantizer in a conventional the delta sigma modulator could be given many levels. This means the quantizer would require many comparators, each of which dissipates power. More importantly, the digital to analog converter in the feedback loop must match the quantizer extremely accurately. This is not readily achieved without expensive techniques, such as trimming. Some of the error may be corrected by digital circuitry on the output of the delta sigma modulator.
Several other configurations have been proposed to improve the dynamic range of an A/D converter. For example, it has been proposed that the levels of the quantizer and the D/A converter in the feedback loop of the delta sigma modulator be exponentially spaced. As an example, the quantizer levels could be 1, 1.5, 1.5 2, 1.5 3, etc. This would achieve a nearly constant signal to noise ratio over a wide variation of input signals. The concept is not practical, as there is no practical way to build a exponential D/A that is also low power and fast. The accuracy demands on the D/A are severe.
A Swiss research company, Centre Suisse d'Electronique et de Microtechnique SA, has shown a "floating point" delta-sigma converter having several different ranges of gain, which switches between the gains as necessary to keep the signal in range. This is difficult to do accurately, as the states in the integrators must be modified when the switch takes place, and a separate circuit must monitor the converter for appropriate times to change gain.
Any converter can be preceded by an analog gain compressor. It is difficult to make this system track, and to achieve low distortion at low current. U.S. Pat. No. 5,289,529 uses an analog compressor before the a/d converter. It has proven difficult to reverse the compression by latter logic well enough to get a true result.
A need remains in the art for a low power A/D converter with sufficient signal to noise ratio and dynamic range at low power.